 Write a list of 10 examples your idea of work.  What do all of these have in common?  What do they require?  How do you assess if work is done?  What is work?  What is the formula?  What are the SI units for work and what do they represent?

 Work is done only when a force moves an object  A force acting on an object and causing it to move a distance is work  Not every force is work..if you push against the wall it does not move..that is not work!  Work = force X distance  Work is measured in Joules  What is a joule exactly? (derive it)  If you pick up a bag of groceries and walk across the room the work is picking up the groceries not the walking.

 The object must move some distance as a result of your force  The force you exert must be in the same direction as the objects motion.  Ie: the groceries  You walk

5  A scientist delivers a speech to an audience of his peers.  A body builder lifts 350 pounds above his head.  A mother carries her baby from room to room.  A father pushes a baby in a carriage.  A woman carries a 20 kg grocery bag to her car?

6 WHAT’S WORK? No  A scientist delivers a speech to an audience of his peers. No Yes  A body builder lifts 350 pounds above his head. Yes No  (Already in her arms) A mother carries her baby from room to room. No Yes  A father pushes a baby in a carriage. Yes No  A woman carries a 20 kg grocery bag to her car? No

 Work can be determined by calculating  Force used x distance moved = amount of work  Therefore what is the formula for work?

 Work = force x distance  Joule – is the SI unit for work.  Newton = force  Meters = distance  Therefore if you exert:  1 Newton of force for 1 meter of distance = 1 joule of work or 1N/m

Work is done when a force is exerted through a distance. A student lifts a bag of books that weighs 135 N. If the bag is lifted.75 m, how much work does the student do? F = 135 Nd =.75 m W = Fd W = (135 N)(.75 m) W = J

COMPLETE PRACTICE PROBLEMS IN BOOK PAGE 285 (1-5) COMPLETE MORE PRACTICE PROBLEMS HANDOUT

 Power tells you how fast something is happening..how fast the work is being done  Power = work/time or Power = Force X Distance Time  Power is measured in watts (W)  One watt is equal to 1 joule per second of work divide joules/seconds

 Power – the rate at which energy is transferred.   P = W P = powerWatts  tW = workJoules  t = timeseconds   1 Watt (W) = 1 J/s 

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 1. WDIK- m = 1500 kg t = 60 sd = 12 m  2. Equations-  P = W/tW = FdF = mg  3. Plug and chug-  F = mg = (1500 kg)(9.8 m/s2) = 14,700 N   W = Fd = (14,700 N)(12 m) = 1.76 x 10 5 J   P = W/t = (1.76 x 10 5 J)/(60 s) = 2940 W 

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Demo: Use a ramp and 4 books and a spring scale and measure distance to move the 200g mass up vertically and horizontally on a ramp Create a data table use books as height w/ 200g hanging mass 1 st Write a hypothesis –more –less- the same-work 2 nd calculate the work for 1. vertically-straight up 2. up the ramp

 Mechanical Advantage – when you increase distance you decrease force but the work remains the same.  Machines –  Multiply force  redirect force- ie: pull down rope –lifts sail  work equation-  force x distance = work

 Machines do not increase the amount of work. They spread out the distance so you don’t have to use the same amount of force to receive the same amount of work.  Prove it:  Work 32 J = work 32 J  Force x distance force x distance  8N x 4 m4 N x 8 m

 Ideal Mechanical advantage = ratio between output force and input force or output distance and input distance without friction  If you have force information use:  Output force /Input force = MA  If you have distance information use:  Input distance/output distance = MA

 Mechanical advantage – multiplying force if you need 3200 N to lift a piano then use a ramp to exert 1600 N of force. 

 OF 3200N = 2 the ramp doubled your  IF 1600N force.  Your output force is 2x your input force.  MA- is 2 no units

 Mechanical Advantage – multiplying distance - you use a ramp that is 6 meters long to raise a piano 3 meters 

 ID- 6 meters = 2 the ramp doubled  OD 3 meters the distance  mechanical advantage of two

 Write a paragraph on what you now know and did it differ from what you knew before,

 Mechanical advantage to machines problem set /answers  hanical%20Advantage.html hanical%20Advantage.html

 An instrument that makes work easier is called a machine  Machines do not have to be complex electrical or gas powered deviced. Even simple objects can be a machine.  A pair of pliers would make it easier to take out a bolt so the pliers would be a machine

 There are two types of work involved in using a machines:  Work that goes into the machine (input)  Work done by the machine (output) Work that comes out of the machine is NEVER greater than the force that is applied to the machine or work that goes into the machine

 Machines make work easier because they change either the size or the direction of the force put into the machine.  Machines multiply either the force or distance to make work easier, but never both!  The comparison of the work output to the work input is called efficiency.  The closer the amount of output is to the amount of input the more efficient the machine is.

 Efficiency is measured in percent and is never more than 100%. This is because the output can never be more than the input  The lower the friction of the machine the more efficient it will be. Keeping a car engine oiled makes it work better and more efficient

 Efficiency – a measure of how much work that is put into a machine is changed to useful work; answer will be a percentage.   efficiency = W out x 100%  W in  W in = work put into the machine  W out = work put out by the machine

 W out = F r d r F r = resistance/output force  d r = resistance/output distance W in = F e d e F e = effort/input force  d e = effort/input distance  For an ideal machine:  W in = W out  F e d e = F r d r

 A worker applies an effort force of 20 N to pry open a window with a resistance force of 500 N. Find the mechanical advantage of thecrowbar. 

 F e = 20 NF r = 500 NMA = ?   MA = F r = 500 N  F e 20 N  MA = 25 

 Find the effort force needed to lift a 2000 N rock, using a jack with a mechanical advantage of 10.

 F r = 2000 NMA = 10F e = ?   MA = F r / F e  F e = F r / MA  F e = (2000 N)/(10)  F e = 200 N

 There are six types of simple machines:  Inclined plane  Wedge  Screw  Lever  Pulley  Wheel and axle

 A ramp is an example of an inclined plane  Simply put in inclined plane is a flat slanted surface  A wedge is an inclined plane that moves and is usually made up of 2 inclined planes  The screw is an inclined plane wrapped around a center bar

 A lever is a rigid bar that pivots or moves around a fixed point. A seesaw is an example  Fulcrum is the fixed point of a lever  A pulley is a rope, belt or chain wrapped around a grooved wheel  A pulley can change the direction of a force or the amount of a force  When you use a pulley you change the direction of the force you are applying.

 A wheel and axle is a simple machine made up of two circular objects of different sizes  The wheel is the larger object the axle is the smaller one  Bicycle is an example of a wheel and axle.. The bike wheel is the large while and the sprocket the chain wraps around is the axle

 /Okeefe/PhySci233/EnergyMachines/Mechani cal%20Advantage.ppt –  s/Notes/Work%20&%20Power/Mechanical%2 0Advantage%20and%20Efficiency.ppt - Similar pagesSimilar pages  education.jlab.org/jsat/powerpoint/0708_simp le_machines_8.ppt -